Most commercially grown Japanese Plum varieties are self-sterile, and thus require cross pollination in order to produce commercial crops. Some varieties such as Santa Rosa, Late Santa Rosa, Beauty and Nubian are partly self-fruitful, but will bare heavier crops with cross pollination. Varieties that produce large amounts of viable pollen and are good pollinizers for other varieties are Santa Rosa, Late Santa Rosa, Wickson, Latoda, and Casselman. The lack of adequate pollination due to poor bloom coordination between the pollinizer variety and the main crop variety, as well as insufficient numbers of pollinizers, is often the most limiting factor in Japanese Plum production. Most Commercially Grown Plum Varieties Require Cross Pollination To Set Commercial Crops. Controlled Pollination can Produce Remarkable Results in CorrectingThese Pollination Problems .

Light Pollinizer Bloom

A light pollinizer bloom will result in the shortage of available pollen to set the main crop trees in the orchard. Pollen can be applied through several means in order to supplement a lack of available pollen in the orchard.

Poorly Coordinated Bloom

Poorly coordinated bloom between the main crop variety and the pollinizer variety can result in a pronounced reduction in the amount of pollen available to set the main crop variety. Plum flowers are usually receptive to pollination from 12 to 48 hours, depending upon environmental and weather conditions. By waiting too long to see if the bloom of the pollinizer variety will coordinate more closely with the main crop variety, growers may be losing that one window of opportunity to use supplemental pollen in order to set a crop that year. Flower receptivity and pollen viability could be lost, and thus there could be insufficient time for the pollen tube to germinate on the stigma, grow down the style and fertilize the ovule; the Effective Pollination Period. By applying pollen when a lack of coordinated bloom is apparent, growers are ensuring that the pollen needed to set a good crop is available.

Poor Weather Condition

Unfavorable weather conditions during the pollination season has always been a problem that tree fruit growers have had to contend with. But what can be done about the weather? Unfortunately, nothing can be done to control the weather, and cool, rainy weather in the during pollination season is the last thing that an orchardist wants. Cool, rainy weather inhibits bee activity and reduces pollen tube growth (Effective Pollination Period) which in turn reduces pollination and fruit set in the orchard. While cool weather may increase the longevity of the ovule, if pollen tube growth is reduced by too great of a factor, then the ovule of the flower will loose it’s ability to set fruit.

During cool spring conditions, when there are breaks in the weather, the grower must be prepared to apply pollen. During some pollination seasons, there may be only a few good hours of pollination weather, when the bees are active and the temperature is warm enough for pollen germination.

Timing the Application

Flowers are most receptive to pollination in the ‘Popcorn’ state of development, just as they are opening. To determine a flowers receptivity to pollination, pick a cherry flower just after it has opened. Grouped in the center of the flower are the yellow anthers. Plum anthers are very small and may require magnification to see clearly. Under magnification the anthers look like a sponge, and the pollen grains are contained upon the surface of this spongy material. When Plum anthers are fresh and the pollen viable they have a bright yellow appearance. After they have dehisced their pollen the anthers look shriveled and brown. In warm and dry conditions, Plum flowers are the most receptive to pollination as soon as they have opened. Under cool, moist conditions, Plum flowers may not be the most receptive until 24 hours after opening.

Bees Are Not Attracted To Certain Types Of Plum Flowers

The fact that honey bees are not attracted to certain varieties of plum flowers creates a major problem for Plum growers in knowing how to approach pollinizing their plum blocks. Bees prefer to forage in many other varieties of blooming plants before turning to forage in Plums. Bees will work plums, but care must be taken when placing honey bee colonies in your blocks of Plums. If possible, any competing flowering plants should be eliminated so that bees will concentrate on working Plum flowers. Make sure that at least 10% of the Plum blossom is open before bees are moved into the orchard, in order to establish the bees working in the Plums and are not lured away by more attractive blossoms elsewhere.